In a differentially pumped mass spectrometer system a sample and carrier gas are introduced to a mass analyser for analysis. One such example is given in FIG. 1. With reference to FIG. 1, in such a system there exists a high vacuum chamber 10 immediately following first, (depending on the type of system) second, and third evacuated interface chambers 11, 12, 14. The first interface chamber is the highest-pressure chamber in the evacuated spectrometer system and may contain an orifice or capillary through which ions are drawn from the ion source into the first interface chamber 11. The second, optional interface chamber 12 may include ion optics for guiding ions from the first interface chamber 11 into the third interface chamber 14, and the third chamber 14 may include additional ion optics for guiding ions from the second interface chamber into the high vacuum chamber 10. In this example, in use, the first interface chamber is at a pressure of around 1-10 mbar, the second interface chamber (where used) is at a pressure of around 10−1-1 mbar, the third interface chamber is at a pressure of around 10−2-10−3 mbar, and the high vacuum chamber is at a pressure of around 10−5-10−6 mbar.
The high vacuum chamber 10, second interface chamber 12 and third interface chamber 14 can be evacuated by means of a compound vacuum pump 16. In this example, the vacuum pump has two pumping sections in the form of two sets 18, 20 of turbo-molecular stages, and a third pumping section in the form of a Holweck drag mechanism 22; an alternative form of drag mechanism, such as a Siegbahn or Gaede mechanism, could be used instead. Each set 18, 20 of turbo-molecular stages comprises a number (three shown in FIG. 1, although any suitable number could be provided) of rotor 19a, 21a and stator 19b, 21b blade pairs of known angled construction. The Holweck mechanism 22 includes a number (two shown in FIG. 1 although any suitable number could be provided) of rotating cylinders 23a and corresponding annular stators 23b and helical channels in a manner known per se.
In this example, a first pump inlet 24 is connected to the high vacuum chamber 10, and fluid pumped through the inlet 24 passes through both sets 18, 20 of turbo-molecular stages in sequence and the Holweck mechanism 22 and exits the pump via outlet 30. A second pump inlet 26 is connected to the third interface chamber 14, and fluid pumped through the inlet 26 passes through set 20 of turbo-molecular stages and the Holweck mechanism 22 and exits the pump via outlet 30. In this example, the pump 16 also includes a third inlet 27 which can be selectively opened and closed and can, for example, make the use of an internal baffle to guide fluid into the pump 16 from the second, optional interface chamber 12. With the third inlet open, fluid pumped through the third inlet 27 passes through the Holweck mechanism only and exits the pump via outlet 30.
In this example, in order to minimise the number of pumps required to evacuate the spectrometer, the first interface chamber 11 is connected via a foreline 31 to a backing pump 32, which also pumps fluid from the outlet 30 of the compound vacuum pump 16. The backing pump typically pumps a larger mass flow directly from the first chamber 11 than that from the outlet 30 of the compound vacuum pump 16. As fluid entering each pump inlet passes through a respective different number of stages before exiting from the pump, the pump 16 is able to provide the required vacuum levels in the chambers 10, 12, 14, with the backing pump 32 providing the required vacuum level in the chamber 11.
The performance and power consumption of the compound pump 16 is dependent largely upon its backing pressure, and is therefore dependent upon the foreline pressure (and the pressure in the first interface chamber 11) offered by the backing pump 32. This in itself is dependent mainly upon two factors, namely the mass flow rate entering the foreline 31 from the spectrometer and the pumping capacity of the backing pump 32. Many compound pumps having a combination of turbo-molecular and molecular drag stages are only ideally suited to low backing pressures, and so if the pressure in the foreline 31 (and hence in the first interface chamber 11) increases as a result of increased mass flow rate or a smaller backing pump size, the resulting deterioration in performance and increase in power consumption can be rapid. In an effort to increase mass spectrometer performance, manufactures often increase the mass flow rate into the spectrometer. Increasing the size or number of backing pumps to accommodate for the increased mass flow rate increases both costs and the size of the overall pumping system required to differentially evacuate the mass spectrometer.
In at least its preferred embodiments, the present invention seeks to provide a compound vacuum pump that can operate more efficiently at higher backing pressures.